Most stocks have too long a LOP, and too Short a Drop At Comb. That means the comb is high- usually intended for shooting a scope sight.
If you are stocking a gun intended to be shot ONLY with iron sights, you use one set of rules. If you are going to shoot a scope, different rules apply.
Assuming this is an iron sighted Traditional Muzzleload, without a globe front sight, or Tang Peep sight, or International Receiver sight mounted on the back of the barrel, most shooters can shoot a stock with a 13 1/2" LOP. Its measured from the trigger you pull to fire the gun( not a set trigger behind it, if you are using a double set trigger arrangement)to the center of the buttstock.
Drop at comb is measured from a line derived by extending a straight edge along the top flat of the barrel, to the rear of the stock. The bottom of that straight edge to the top of the comb, at its forward edge, is where the the DROP AT COMB is measured. Typically, it is 1 1/2" to 2 1/2" in length, depending on the shooter's face, and the distance from the center of his pupil, looking straight ahead, to the bottom of his cheekbone.
If the shooter has a normal neck size, then the Drop at Heel, at the top of the buttplate, is going to be 2 1/2 to 3 3/4" in length. But if you have a neck that is long- measure from your collar bone to the jaw- then you might be more comfortable with a slightly longer DROP at HEEL. I have one gunstock that is very comfortable for me to shoot off-hand with a 4 inch DROP AT HEEL.
And that gets me to my last comments. If you are building a gun designed to be shot prone, like a " Chunk Gun", then you want the stock to be " Straighter", with a shorter DROP AT HEEL, and even a smaller DROP AT COMB. Because of the nature of the position, a LONGER LOP may also be in order to protect you from recoil.
Most shooters would be better served spending more time looking at Cast On( Lefties) or Cast Off for their stocks. Too many shooters are seen laying their cheeks on the top of the comb, to get the sights aligned with their shooting eye, rather than placing the stock next to their cheek.
Do try other guns to see what seems to fit you the best. Then measure them. Take a tape measure to a gun show, or well stocked gunshop, and go through the used gun rack to get a feel for the stocks on them. There are always vertical and horizontal surfaces that you can lay the gun barrels and sights against to find that line to measure drop. Just subtract the height of the top of the sights over the barrel. CAST OFF will not only let you align your shooting eye with your sights, but it also directs recoil away from your face.